The present invention relates to a method for the combined production of hydrogen and carbon dioxide from a hydrocarbon mixture, in which the hydrocarbon mixture is reformed to produce a syngas which is cooled, then enriched with H2 and CO2, optionally dried, and treated in a PSA hydrogen purification unit to produce hydrogen, the offgas being treated to capture the CO2.
Climate change is one of today's major environmental problems. The increase in the concentration of greenhouse gases in the atmosphere, particularly carbon dioxide, is one of its essential causes. One of the major challenges facing mankind today is to decrease greenhouse gas emissions, and in particular to decrease CO2 emissions.
CO2 produced by man originates from many sources, and each type of emission must be decreased. However, one of the essential emissions is that generated by the combustion of fuels, especially fossil fuels.
The European Community is committed to achieving an 8% reduction of its greenhouse gas emissions between 2008 and 2012, compared to the 1990 level. To help obtain this result, directive 2003/87/EC establishes an emission trading system (ETS) for greenhouse gas emissions. Thus industrial facilities must buy quotas corresponding to their greenhouse gas emissions, and particularly carbon dioxide.
Plants producing hydrogen and carbon monoxide emit carbon dioxide by burning fuel. The CO2 contained in the flue gases originates from the combustion of valueless gases generated in the process and recycled in the form of fuel, and from additional fuels such as naphtha and natural gas.
While plants producing H2/CO are not yet concerned, they will be included in the ETS from 2013 onwards.
Moreover, alongside directive 2003/87/EC—which is only concerned with European Union countries (where the ETS will be applied)—another requirement has recently emerged, concerning all countries: applications for permits to operate industrial facilities may be dependent on the capacity of the facility to capture the CO2 emissions.
Thus, since they will shortly be subject to both these requirements, such facilities, and hydrogen plants in particular, must forthwith develop high yield CO2 capture solutions.
CO2 capture operations capture the CO2 present in the combustion flue gases, and also the CO2 present in the process gas.
It is the object of the invention to improve this second type of CO2 capture operation.
CO2 capture solutions on process gas that are currently feasible—technically and/or financially—only succeed in capturing up to 60% of the CO2 present. A characteristic example is the amine (MDEA) scrubbing technology on high-pressure syngas.
Let is now consider the particular case of hydrogen production.
In order to produce hydrogen from a hydrogen-rich gas, typically a syngas, the process used to separate and purify the hydrogen is the pressure swing adsorption (PSA) process. This process generates a stream of pure hydrogen, generally having a purity above 99% by volume, and a hydrogen-lean offgas containing the other species present in the initial mixture to be purified, including the CO2.
One solution currently used to capture the CO2 consists in recovering it from the offgas from the hydrogen purification unit, via a compression and purification unit (CPU).
At present, optimizing the operation of a hydrogen purification unit by pressure swing adsorption—also called H2 PSA—means seeking to achieve two objectives:                to obtain a given hydrogen purity (for example containing a maximum of a few ppm of CO2), and        to obtain a high total yield of hydrogen product by the unit.        
However, since an improved CO2 capture solution needs to be adopted in order to take account of this new requirement on limiting CO2 emissions, a hydrogen production process will therefore also have to integrate a CO2 capture process, having a high CO2 capture yield.
Therefore, the problem facing a person skilled in the art is to reconcile a high hydrogen yield with an equally high CO2 capture yield.